Hand-held power-tools such as, but not limited to, chain saws and line trimmers, are often powered by small two stroke internal combustion engines that are equipped with diaphragm carburettors.
Generally, a diaphragm carburettor has an air passage where fuel and air is mixed in a correct ratio. An outlet of the air passage leads to a crankcase of the engine. Typically, a throttle valve is provided in the air passage to control the amount of fuel and air mixture that enters the crankcase.
Usually, before starting an engine either a purging system or a priming system is actuated at least once to introduce fresh fuel into the carburettor. Typically, the purging system is used to remove residual air or fuel from the carburettor and fill desired fuel passages and chambers of the carburettor with the fresh fuel. On the other hand the priming system is used to inject a small quantity of fuel into the air passage, often in addition to performing the functions of a purging system. The fresh fuel supplied to the carburettor before starting the engine helps in an easy and quick starting of the engine. The purging system and the priming system are typically actuated by a purge bulb and a primer bulb respectively.
Further, the engines are also provided with a fuel enrichment system which is actuated in order to achieve a rich mixture of air and fuel (more fuel to air) during a cranking of the engine. The fuel enrichment system works by supplying extra fuel during the cranking of the engine, which facilitates a stable starting of the engine. One kind of a fuel enrichment system is the choke system. The choke system may include a butterfly or a slide valve located at the entrance of the carburettor (internal choke system). The valve can be moved between multiple positions via a lever, in order to control the air flow into the carburettor. The choke system may also be a separate system outside the carburettor (external choke system). The choke system is used to create an increased vacuum in the air passage, which draws extra fuel from fuel circuits of the carburettor. Another kind of fuel enrichment system includes a fuel enrichment circuit, which can be actuated by pressing a push button to introduce extra fuel into the air passage as soon as the engine cranking cycles are started. Sometimes both a choke system and a fuel enrichment circuit are used to enrich the fuel and air mixture.
To achieve an easy and quick starting of the engine followed by a stable cranking (typically by pulling a rope) may require actuation of multiple systems, for example a purging system or a priming system, and the choke valve and/or the fuel enrichment circuit. Therefore, the starting procedure for this kind of engines usually consists of three steps: 1) depressing a purge or primer bulb at least once, 2) actuating a fuel enrichment system and 3) pulling a rope to start the engine.
U.S. Pat. No. 7,334,551 issued on Feb. 26, 2008 to George M. Pattullo, titled “Combustion engine pull cord start system” describes a two-step starting system. In the two-step starting system taught by this patent, a pull-rope system of an internal combustion engine is linked to a throttle valve and a choke valve. The choke valve is actuated in different positions by a rope tension of the pull-cord system. However, the design is quite complex with many moving parts.
In light of the foregoing, there is a need for an improved two-step starting system, for an internal combustion engine, having a simple design with a lower number of moving parts.